Calcium is the chemical element in the periodic table that has the symbol Ca and atomic number 20. Calcium is a soft grey alkaline earth metal that is used as a reducing agent in the extraction of thorium, zirconium and uranium. Calcium is also the fifth most abundant element in the Earth's crust. It is essential for living organisms, particularly in cellphysiology, and is the most common metal in many animals.

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Calcium is a rather soft, gray, metallic element that can be extracted by electrolysis from calcium fluoride. It burns with a yellow-red flame and forms a white nitride coating when exposed to air. It reacts with water, displacing hydrogen and forming calcium hydroxide.

In the U.S., between about 50% and 75% of adults do not get sufficient calcium in their diet.[1] Adults need between 1,000 and 1,300 mg of calcium in their daily diet.[1]

The most abundant isotope, 40Ca, has a nucleus of 20 protons and 20 neutrons. Its electron configuration is: 2 electrons in the K shell (principal quantum number 1), 8 in the L shell (principal quantum number 2), 8 in the M shell (principal quantum number 3), and 2 in the N shell (principal quantum number 4). The outer shell is the valence shell, with 2 electrons in the lone 4s orbital, the 3p orbitals being empty.

Calcium is not naturally found in its elemental state. Calcium is found mostly in soil systems as limestone, gypsum and fluorite.
Stalagmites and stalactites contain calcium carbonate. Being an essential macromineral in the human diet, soil conservation practices often consider the sustainable equilibrium of calcium concentrations in the earth.

Calcium is an important component of a healthy diet. A deficit can affect bone and tooth formation, while overretention can cause kidney stones. Vitamin D is needed to absorb calcium. Dairy products, such as milk and cheese, are a well-known source of calcium. However, some individuals are allergic to dairy products and even more people, particularly those of non-European descent, are lactose-intolerant, leaving them unable to consume dairy products. Fortunately, many other good sources of calcium exist. These include: seaweeds such as kelp, wakame and hijiki; nuts and seeds (like almonds and sesame); beans; seafood such as oysters and shrimp; soft-boned fish; amaranth; whole wheat; collard greens; okra; rutabaga; broccoli; and fortified products such as orange juice and bread.

Calcium (Latin calcis, meaning "lime") was known as early as the first century when the Ancient Romans prepared lime as calcium oxide. It was not actually isolated until 1808 in England when Sir Humphrey Davy electrolyzed a mixture of lime and mercuric oxide. Davy was trying to isolate calcium and when he heard that Berzelius and Pontin prepared calcium amalgam by electrolyzing lime in mercury, he tried it himself.

Calcium has four stable isotopes (40Ca and 42Ca through 44Ca), plus two more isotopes (46Ca and 48Ca) that have such long half-lives that for all practical purposes they can be considered stable. It also has a cosmogenic isotope, radioactive 41Ca, which has a half-life of 103,000 years. Unlike cosmogenic isotopes that are produced in the atmosphere, 41Ca is produced by neutron activation of 40Ca. Most of its production is in the upper metre or so of the soil column where the cosmogenic neutron flux is still sufficiently strong. 41Ca has received much attention in stellar studies because it decays to 41K, a critical indicator of solar-system anomalies.

97% of naturally occurring calcium is in the form of 40Ca. 40Ca is one of the daughter products of 40K decay, along with 40Ar. While K-Ar dating has been used extensively in the geological sciences, the prevalence of 40Ca in nature has impeded its use in dating. Techniques using mass spectrometry and a double spike isotope dilution have been used for K-Ca age dating.

There are conflicting recommendations about when to take calcium supplements. However, most experts agree that no more than 500 mg should be taken at a time – any excess will go to waste. It is recommended to spread doses throughout the day, with the last dose near bedtime. Recommended daily calcium intake varies from 1000 to 1500 milligrams, depending upon the stage of life.

Calcium carbonate is the most common and least expensive calcium supplement. It can be difficult to digest and causes gas in some people. Taking magnesium with it can help to prevent constipation. Calcium carbonate is 40% elemental calcium. 1000 mg will provide 400 mg of calcium. Take this supplement with food to aid in absorption.

Calcium citrate is more easily absorbed (bioavailability is 2.5 times higher than calcium carbonate), easier to digest and less likely to cause constipation and gas than calcium carbonate. It also has a lower risk of contributing to the formation of kidney stones. Calcium citrate is 21% elemental calcium. 1000 mg will provide 210 mg of calcium. It is more expensive than calcium carbonate and more of it must be taken to get the same amount of calcium.

Calcium phosphate costs more than calcium carbonate, but less than calcium citrate. It is easily absorbed and is less likely to cause constipation and gas than either.

Calcium lactate and calcium aspartate are both more easily digested, but more expensive than calcium carbonate.

Milk, yogurt, cheese and other dairy products are a prime source of calcium and are also fortified with vitamin D. Calcium is the most abundant mineral in the body, but it is also the one most likely to be inadequately supplied in the diet.

Calcium is essential for the normal growth and maintenance of bones and teeth, and calcium requirements must be met throughout life. Requirements are greatest during periods of growth, such as childhood, during pregnancy and when breast-feeding. Long-term calcium deficiency can lead to osteoporosis, in which the bone deteriorates and there is an increased risk of fractures. Calcium needs can be met by eating or drinking at least three or four servings of dairy products daily.

Some dairy products, such as hard cheese and whole milk, do contain a significant amount of saturated fat, which can contribute to cardiovascular disease. Therefore, a diet consisting of low-fat dairy products should be considered. A low-fat variety of cheese made with part-skim milk, such as mozzarella, ricotta, cottage or farmer's cheese might be chosen.

For those with an intolerance or allergy to dairy products, substitutes such as soya or rice milk often have calcium added to them.